The present invention is directed, in general, to power supplies and, more specifically, to a control circuit for paralleling power supplies and a method of operating the same.
A power supply is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a stable and well-regulated output, switched-mode power supplies are frequently employed to advantage. Switched-mode power supplies generally include an inverter, a transformer having a primary winding coupled to the inverter, an output circuit coupled to a secondary winding of the transformer, an output filter and a controller. The inverter generally includes a power switch, such as a field-effect transistor (FET), that converts an input voltage to a switched voltage that is applied across the transformer. The transformer generally transforms the voltage to another value. The output circuit generates a desired voltage at the output of the power supply and typically includes an output capacitor that smooths and filters the output voltage for delivery to a load.
In conventional voltage or current mode control, the switching cycle may be divided into a primary interval D (during which the power switch is in the conduction mode) and an auxiliary interval 1-D (during which the power switch is in the non-conduction mode). The modulator, in concert with internal timing and error signals derived from either a voltage or current being monitored, determines when the power switch is conducting or non-conducting. Variations in the error signal serve either to prolong the conduction mode or advance initiation of the non-conduction mode.
Conventional power supplies may provide an output over-current protection capability that keeps their output current from exceeding a rated output current. For example, a conventional power supply may provide a DC output voltage of 48 volts for a DC output current up to a rated value of 12.5 amperes, thereby yielding a maximum rated output power of 600 watts. For this example, as the output current increases beyond 12.5 amperes, the conventional power supply reduces its output voltage while trying to maintain a constant output current of 12.5 amperes.
Component tolerance differences that exist from one power supply to another power supply result in somewhat different values of output currents at the point where over-current protection begins. Therefore, conventional over-current protection schemes are designed from a worst-case standpoint to ensure that power limits are not exceeded. Ideally, the output voltage of a power supply should decrease rapidly to, and remain at, zero as the over-current protection scheme is in operation.
Another problem that occurs in some conventional approaches is that, while the output voltage may initially decrease sharply for a constant output current, it reaches a point above zero volts where the output current begins to increase. The value of this voltage plateau varies due to both component tolerances and over-current protection circuit designs. If this voltage plateau is too high or its slope is too flat, the power and current capability of the power supply can be exceeded.
One approach to increase the output current capability that a single power supply can deliver at a rated voltage is to operate several power supplies in a parallel mode and share the output current load between them. Paralleling of two power supplies to share output current requires that the outputs of the power supplies be conjoined. Unless the output voltages are essentially equal, the power supply with the higher voltage setpoint will attempt to deliver the entire load current until it reaches its output current limit. At this point, the output voltage of this power supply having the initial higher voltage setpoint would be reduced to a point approximately equal to the lower output voltage setpoint of the other power supply. This action is undesirable and may cause a power limit to be exceeded due to a wide variation in current limit setpoints between units.
A conventional approach to resolving this problem involves raising the power supply""s output impedance to allow its output voltage to fall below its normally regulated value (xe2x80x9cdroopxe2x80x9d) as its output current increases. Component tolerances dictate both the magnitude and the droop of the output voltage for each power supply. In parallel operation, the output current is provided by the power supply having the greatest voltage magnitude. This occurs until the output current demand causes its output voltage to droop toward a value equalling that of a second power supply output voltage that is not yet providing any output current. At this point, the second power supply begins to contribute to the output current; the process continues until all paralleled power supplies contribute some output current to the whole. While this approach allows power supplies to be operated in a parallel mode, excessive output voltage droop may cause the combined output voltage to vary more than is tolerable.
Accordingly, what is needed in the art is a way to simplify the parallel operation of power supplies and provide a better-regulated output voltage.
To address the above-discussed deficiencies of the prior art, the present invention provides, a control circuit for, and method of, allowing the power supply, having a controller and an output current limit, to be operated in parallel with other power supplies and a power supply incorporating the circuit or the method. In one embodiment, the circuit includes: (1) a voltage sense subcircuit, coupled to an output of the power supply, that produces a voltage control signal that is a function of an output voltage of the power supply and a voltage proportional to an output current of the power supply and (2) a current sense subcircuit, coupled to the power supply, that produces a current sense signal that is a function of the output current of the power supply, a combination of the voltage control signal and current control signal employable to modify an output of the controller and thereby regulate said power supply and allow said power supply to continuously operate in a current limit region.
The present invention introduces, in one aspect, the broad concept of improving the paralleling of power supplies by basing the output voltage control signal on a voltage proportional to the output current of the power supply, rather than solely on a voltage of the output of the power supply. Thus, the control circuit uses the voltage proportional to the output current to provide a more precise droop regulation via the voltage sense subcircuit. Additionally, a more precise determination of a specified maximum output current value, via the current sense subcircuit, allows a more predictable maximum output current value for the power supply. The combination of the more precise droop regulation and predictable maximum output current value facilitates enhanced paralleling of multiple power supplies.
In one embodiment of the present invention, the power supply further includes an isolation transformer having a secondary winding. The voltage control signal is further configured to be a function of a voltage at the secondary winding. The control circuit further improves the paralleling of power supplies by basing the output voltage control signal on the more stable voltage present at the secondary winding of the power supply. This voltage at the secondary winding provides an appropriate supply voltage to both the voltage sense subcircuit and the current sense subcircuit to serve as a bias voltage even when the output voltage of the power supply is below its rated output value.
In one embodiment of the present invention, the voltage sense subcircuit comprises a rectifier that rectifies the voltage at the secondary winding. In an embodiment to be illustrated and described, the rectifier takes the form of a single diode.
In one embodiment of the present invention, the voltage sense subcircuit comprises an error amplifier and employs the voltage at the secondary winding as a bias supply voltage for the error amplifier.
In one embodiment of the present invention, the current sense subcircuit comprises a resistor coupled to the power supply. In a related embodiment, the current sense subcircuit comprises a sensor that forms a portion of a current return path in the power supply. In an embodiment to be illustrated and described, the current sense subcircuit comprises a resistor that forms a portion of a current return path in the power supply.
In one embodiment of the present invention, the circuit further includes first and second isolation diodes coupled to outputs of the current sense subcircuit and the voltage sense subcircuit, respectively. In an embodiment to be illustrated and described, the first and second isolation diodes provide the current sense signal and voltage sense signal to a pulse-width modulator that forms a portion of the controller for the power supply.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.